1. Field of the Invention
The present invention relates to a manufacturing method of a semiconductor apparatus including a semiconductor chip and an electrode which is formed on the semiconductor chip and electrically connected with an external lead and, particularly, to a manufacturing method of a semiconductor apparatus capable of carrying a large current.
2. Description of Related Art
Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs), which are used for power supply in consumer-electronics products such as cell phones, personal computers, and digital audiovisual equipments or for driving a motor of vehicles, carry a high current of about 5 to 200 A. It thus requires a thick, heavy wire material. For example, a plurality of large-gauge wires of about 100 to 500 μm are connected to the source electrode of a MOSFET in order to draw a high current.
However, the connection of a bonding wire to the source electrode by ultrasonic bonding or thermocompression bonding requires a bonding area of about 1.5 to 3 times the wire cross-sectional area. The area of the source electrode or the gate electrode, however, is limited to small and thus subject to the limitation by the wire cross-sectional area. Further, high pressure application is required to obtain a large bonding strength, and the mechanical impact causes a decrease in yields. This is described in Japanese Unexamined Patent Application Publication No. 2002-313851 (Oono), for example.
In a semiconductor apparatus disclosed in Oono, a plate-shaped electrical path member is bonded by ultrasonic to thereby obtain a large bonding strength and achieve high yields. FIG. 8 is a bird's-eye view of a semiconductor apparatus (Small Out-line Package (SOP)—8 packages) taught by Oono. FIGS. 9A and 9B are cross sectional views along lines IVA-IVA and IVB-IVB, respectively, in FIG. 8.
Referring to FIG. 8, the MOSFET 101 is almost entirely fixed and covered by a sealing resin (molding resin) 102 which is made of epoxy resin or the like. The MOSFET 101 has eight leads 103, such that one end of each lead 103 is exposed outside the molding resin 102.
Referring then to FIG. 9A, the molding resin 102 includes a semiconductor device (semiconductor chip) 105. On the semiconductor device 105, a source electrode (source pad) 104s and a gate electrode (gate pad) 104g are formed on the top surface, and a drain electrode (drain pad), though not shown, is formed at the end of the under surface.
Four terminals arranged on one side out of the terminals of the eight leads 103 are integrated into one set inside the molding resin 102. The four terminals serve as drain-side terminals 103d of the leads 103 which are to be electrically connected with the drain pad. The remaining four terminals out of the terminals of the eight leads 103 are arranged so as to avoid direct contact with the semiconductor device 105 and to be electrically disconnected from the drain-side terminals 103d and the lead 103 including a drain-side post 107d inside the molding resin 102 as shown in FIG. 9A. Out of the remaining four terminals of the leads 103, three terminals are integrated into one set, and the remaining one terminal is electrically disconnected from those three-in-one terminals of the lead 103.
The thee-in-one terminals of the lead 103 are electrically connected with the source electrode 104s of the semiconductor device 105 by a current path member 106 to serve as source-side terminals 103s of the leads 103. The remaining one terminal of the lead 103 is electrically connected with the gate electrode 104g of the semiconductor device 105 by a single bonding wire 108 to serve as a gate-side terminal 103g of the lead 103.
Referring to FIGS. 9A and 9B, in the current path member 106, an electrode-side connecting portion 106a which is formed at one end is connected by plane with the source electrode 104s, and a lead-side connecting portion 106b which is formed at the other end is connected by plane with a source-side post 107s. The current path member 106 is directly connected to both the source electrode 104s and the source-side post 107s of each source-side terminal 103s of the leads 103 at the same time by ultrasonic bonding.
With the current path member 106, the semiconductor apparatus 101 taught by Oono allows the cross sectional area of the current path flowing between the source electrode 104s of the semiconductor device 105 and the source-side post 107s of each source-side terminal 103s of the leads 103 to be significantly larger than a total of the cross sectional area of the current path flowing through a plurality of bonding wires in a conventional MOSFET. This enables the MOSFET 101 to have significantly lower resistance between the source electrode 104c and the lead 103 compared with a conventional MOSFET.
As a different bonding technique from the ultrasonic bonding, there is laser bonding which uses laser as disclosed in Japanese Unexamined Patent Application Publication No. 1-310547 (Iino), for example. Further, Japanese Unexamined Patent Application Publication No. 6-244230 (Uemura et al.) discloses a bonding technique using both ultrasonic and laser.
However, the ultrasonic bonding described in Oono and Uemura et al. cause damages to products because it is a mechanical bonding technique and also cause oxidation of wires (connecting member) or electrodes because it uses heat.
The bonding process may be carried out at the temperature of about 300° C., for example, so that an oxide film is formed on metal. Further, the temperature in the bonding process is low compared with the melting point of a metal such as a wire member, which is 800° C. to 1100° C. Thus, a mechanical energy is required in order to break the oxide film formed on a wire member and to enable bonding with the low temperature in the bonding process between the electrode and the wire material. In such a case, mechanical vibration is applied so as to break the oxide film and expose a new surface, which cause damage to the chip. Particularly, a MOSFET has an active cell below the pad, and the damage caused by the ultrasonic vibration can reach the element below the pad, which leads to breakdown of a product. Further, because the process forms an alloy by mechanical contact, the bonding is not appropriate for some types of metal.
Further, the technique of laser bonding can cause damage to a base below the electrode. As described above, large-gauge wires of about 100 to 500 μm or above are used to reduce the resistance in order for the MOSFET to carry a high current. This corresponds to the thickness of 100 to 500 μm. On the other hand, the thickness of the electrode to be connected is as small as about 2 to 6 μm as described in Oono. The adjustment of laser intensity is extremely difficult when laser-welding the members having different thicknesses, and it is impossible to provide bonding by the laser welding. High laser intensity to fuse the wire causes breakdown of the base, and low laser intensity fails to establish connection or obtain desired connection intensity.